This invention relates to the production of silicon carbide, and more particularly this invention relates to an electric resistance furnace for the production of silicon carbide.
Industrial silicon carbide is produced by a discontinuous process in an electric resistance furnace, which process was originally devised by Acheson. In those resistance furnace installations operated by direct electric resistance heating and that can be used exclusively for pure solids reactions, the current is supplied by electrodes through a resistance core of carbon-containing material that is horizontally embedded in the burden consisting of a mixture of granulated coke, quartz sand and additive materials. The electric current effects in the horizontal material column a pure resistance heating, the reaction of materials taking place in the solid phase, that is, in making silicon carbides, a diffusion reaction develops within the temperature range of from about 1500.degree. to 2500.degree. C., preferably from about 1700.degree. C. to 2500.degree. C.
Resistance furnaces of known construction are in general reactangular open at the top and up to 20m long. The bottom and the solid front walls are bricked up with refractory bricks while the side walls are removable. The current is supplied by electrodes embedded in the front walls (see Ullman's Encyclopedia of Industrial Chemistry, Volume 3, 4th edition 1973, pages 534 and following Chapter: Resistance Furnaces).
According to a more recent embodiment of those resistance furnaces as shown in U.S. Pat. No. 3,950,602, the electrodes can also be disposed as bottom electrodes connected to the resistance core by an electrically conductive material, said connection not being built as a component of the resistance core and having higher electric conductivity than the latter. The current is supplied by connection to electric circuits beneath the floor. The burden needed for the reaction can be charged via the bottom electrodes and the resistance core in conformity with its natural alluvial cone and the installation can be operated as a mound furnace without walls, that is, without lateral and front boundaries of wall elements. But the frame installation as a whole can be also surrounded in a conventional manner with walls that receive the burden, but here it is possible to use simple, transportable walls both for the lateral boundary and for the front closure. The open ballast is of course not profitable in a shed due to the large amount of space needed, wherefore those furnaces are at best operated as stationary outdoor installations. As bottom electrodes one can use graphite and/or carbon electrodes equipped with current and cold water lines, said electrodes being usually employed in furnace installations with the so-called front electrode arrangement. Coke and/or graphite equipped with current and cold water lines can also be used for these stamped electrodes, as well as metal electrodes, for as a result of the increased spatial distance between the electrodes and the heating zone proper, the temperatures that appear in the electrodes are considerably lower than in the known furnace installations with electrodes embedded in the front end.
In another embodiment according to U.S. Pat. No. 3,989,883, those installations can also be operated with a combined arrangement of the electrodes, for example, a front electrode and an electrode arranged as a bottom electrode.
However, in all installations of the known constructions, the resistance core embedded in the burden is horizontally disposed in the longitudinal direction, thatis, that is, the form of a single horizontal material column between the electrodes, the spacing between the two electrodes being determined by the given length of the resistance core. Accordingly, the silicon carbide roll formed after termination of the heating phase appears in the form of an elongated cylinder.
It is true that a few tests have been disclosed that differ from the arrangement of the resistance core in the form of a single elongated horizontal material column. Thus, for example, in German Pat. No. 160,101 two parallel, adjacent, elongated power cores are used between two front electrodes. But this arrangement serves not for the production of silicon carbide but for so-called silicon oxycarbides formed by an undersupply of carbon and at a temperature insufficient for the formation of silicon carbide.
In accordance with U.S. Pat. Nos. 941,339 and 1,044,295 there is recommended that the resistance core be disposed in zigzag fashion between the front electrodes, whereby the losses of heat are to be reduced by radiation, and according to German Pat. No. 409,356, an annular heating core is used in association with a spheroidal configuration of the furnace body, electrodes being introduced in the periphery of the sphere. The end product is to assume here the configuration of a cake from the mold of a flat spheroid. But none of these ideas has ever achieved industrial significance.
In all furnace installations of the known construction, the current path is given from the transformer via the first electrode through the resistance core via the second electrode and from the latter through a so-called secondary circuit back to the transformer, it being necessary to locate the secondary circuit as close as possible to the furnace area to achieve a favorable power factor that depends on the size of the surface enclosed by the current path. The secondary circuit is usually laid beneath the furnace bottom, that is, under the floor, so that it cannot be damaged or destroyed by mechanical apparatus during the charging and dismounting of the furnace or by the corrosive action of the hot reaction gases and by so-called "blowers" during the heating phase. For an effective protection against the high temperatures that the furnace bottom can reach during the heating phase, expensive cooling means are needed for the secondary circuit, and in addition the circuit laid under the floor is accessible only with great difficulty when this is required by an interruption in the operation of the furnace. Besides, in case of a thermal destruction of the cool water jacket during the heating phase, there is the danger that the cool water can penetrate the furnace area causing possible explosive reactions.